To obtain insight into the glycoprotein nature of S-antigen (S-Ag), we have investigated the affinity of bovineS-Ag for plant lectins, location of the sugar moiety within the molecule, and incorporation of radiolabelled mannose and glucosamine into S-Ag. It was found (i) that only about 10% of purified S-Ag was bound to concanavalin A (Con A) and wheat germ agglutinin (WGA) columns, (ii) that both concanavalin A and wheat germ agglutinin bound chymotryptic peptides derived from the C-terminal half of S-Ag, and (iii) that radiolabelled D-mannose and D-glucosamine were incorporated into S-Ag and the incorporation was inhibited by tunicamycin, an N-glycosylation inhibitor.14C-Mannose-labelled S-Ag was identified by affinity chromatography on an anti-S-Ag antibody column. These results support the supposition that only a small population of S-Ag is glycosylated (probably in N glycosylated form), and the sugar moiety is located in the C-terminal half of the molecule.